In LTE Release-11, discontinuous reception (DRX) is used as the time domain multiplexing (TDM) solution to solve in-device coexistence problem. In UE assistance information sent from UE to eNB, UE can report DRX starting offset, which is useful to reduce or avoid WiFi beacon collision. However, reporting a single DRX starting offset value has the problem that there is restriction on eNB scheduling flexibility. For example, if eNB already uses the same DRX starting offset for many other UEs, then using the same DRX starting offset results in that many subframes are overloaded while other subframes are underloaded.
In LTE Release-11, Enhanced Physical Downlink Control Channel (EPDCCH) is introduced. In E-PDCCH, each DCI is transmitted over one subframe. This is different from PDCCH which transmits within a first few symbols in a subframe. User equipment (UE) may monitor UE specific search space in ePDCCH when ePDCCH is configured. However, the UE also monitors common search space in PDCCH.
In LTE, an inactivity timer is started at the subframe when the UE receives initial DL and UL grant and counts from the next subframe. The PDCCH decoding may finish before the next subframe. However, if ePDCCH is introduced, the UE starts ePDCCH decoding in the end of the subframe because the UE needs to receive the subframe to decode the ePDCCH DCI message. Even if the UE decoding time is very short, the UE would not be able to complete the ePDCCH decoding in the next subframe at the earliest. Therefore, the UE could not determine whether the initial DL or UL grant is received in the next subframe. Due to the latency with ePDCCH decoding, the UE may not able to start the inactivity timer at the subframe.
If the next subframe is an active subframe, there is no problem even if the UE cannot start inactivity timer in the subframe in which the UE receives initial DL or UL grant. However, if the next subframe is an inactive subframe, the UE cannot monitor PDCCH or ePDCCH even if the active time is extended with inactivity timer because the decoding in ePDCCH has not completed.